1. Field of the Invention
The present invention relates to a photodetector apparatus which receives an optical signal having arrived there with a photodiode, so as to detect the intensity of optical signal.
2. Related Background Art
A photodetector apparatus comprises a photodiode and an integrating circuit, in which the photodiode converts inputted optical signals into current signals and outputs thus obtained current signals, and the integrating circuit inputs and integrates the current signals outputted from the photodiode, thereby outputting a voltage signal. According to the integral signal outputted from the integrating circuit, the intensity of optical signal is detected.
The integrating circuit in a conventional photodetector apparatus usually comprises a 2-input/1-output differential amplifier. Namely, the photodiode is connected to the first input terminal of differential amplifier, the second input terminal of differential amplifier is set to a reference potential, and a capacitor and a switch are disposed in parallel between the first input terminal and output terminal of the differential amplifier. In this integrating circuit, the output of differential amplifier is initialized when the switch is closed, and then the switch is opened for a predetermined period of time, so that current signals outputted from the photodiode are stored as an electric charge, whereby the voltage signal corresponding to the electric charge stored in the capacitor is outputted.
Also known are photodetector apparatus (solid-state imaging apparatus) in which photodiodes are arranged one- or two-dimensionally, so as to be able to detect (capture) a spatial intensity distribution of inputted optical signals, i.e., an image (see, for example, Japanese Patent Application Laid-Open No. HEI 9-270961 and 10-4520). Such a photodetector apparatus comprises individual integrating circuits corresponding to respective photodiodes or respective columns of photodiodes, whereas each integrating circuit has a configuration such as the one mentioned above.
Such a photodetector apparatus is used as a radiation detector in a radiation CT apparatus, for example. In this case, signals outputted from the photodetector apparatus are required to have a very high S/N ratio, and noise is needed to be reduced to the limit. However, the conventional photodetector apparatus have failed to output signals with a sufficient S/N ratio.
In order to overcome the problem mentioned above, it is an object of the present invention to provide a photodetector apparatus which can output signals with an excellent S/N ratio.
This photodetector apparatus comprises a full differential amplifier having two input terminals and two output terminals; a photodiode connected to one of the input terminals; a first capacitor and a first switch which are connected in parallel between one of the input terminals and one of the output terminals; a second capacitor and a second switch which are connected in parallel between the other of the input terminals and the other of the output terminals; and a differential amplifier circuit connected to both of the output terminals.
More preferably, this photodetector apparatus is a photodetector apparatus comprising a photosensitive section which has a photodiode for converting an inputted optical signal into a current signal and outputs the current signal, the photodetector apparatus comprising: (A) an integrating circuit including a first full differential amplifier, having first and second input terminals and first and second output terminals, for feeding the current signal from the photosensitive section into the first input terminal; an additional capacitor, having a capacitance substantially equal to a junction capacitance of the photodiode, connected to the second input terminal of the first full differential amplifier; a first capacitor disposed between the first input terminal and first output terminal of the first full differential amplifier; a first switch disposed in parallel with the first capacitor; a second capacitor disposed between the second input terminal and second output terminal of the first full differential amplifier; and a second switch disposed in parallel with the second capacitor; the integrating circuit inputting and integrating the current signal outputted from the photosensitive section, and outputting integral signals corresponding to a result of integration from the first and second output terminals of the first full differential amplifier, respectively; and (B) a differential amplifier circuit for inputting the respective integral signals outputted from the first and second output terminals of the first full differential amplifier in the integrating circuit and outputting, according to a difference therebetween, a signal corresponding to an intensity of the optical signal.
In the photodetector apparatus, an inputted optical signal is converted into a current signal by the photodiode of the photosensitive section, and this current signal is fed into the first input terminal of the first full differential amplifier in the integrating circuit. If the first and second switches are open, then the inputted current signal is stored as an electric charge in the first capacitor. Since the additional capacitor having a capacitance substantially equal to the junction capacity of the photodiode is connected to the second input terminal of the first full differential amplifier, the first output terminal of the first full differential amplifier outputs an integral signal corresponding to the electric charge stored in the first capacitor, and the second output terminal of the first full differential amplifier outputs an integral signal corresponding to the one obtained when the polarity of the electric charge is reversed. The respective integral signals outputted from the first and second output terminals of the first full differential amplifier are fed into the differential amplifier circuit, and the latter outputs, according to the difference between these signals, a signal corresponding to the intensity of optical signal. The signal outputted from the differential amplifier circuit is only the one corresponding to the magnitude of the current signal outputted from the photosensitive section since noise components cancel each other out, thus yielding an excellent S/N ratio.
In the photodetector apparatus, the integrating circuit may comprise first reference potential setting means for setting each of the first and second output terminals of the first full differential amplifier to a reference potential before an integrating operation. In this case, even though the respective potentials of two output terminals in the first full differential amplifier may be unstable, each of the integral signals outputted from the first full differential amplifier becomes stable.
In the photodetector apparatus, the photosensitive section may have photodiodes arranged in an array of M rows and N columns and sequentially output respective current signals from N photodiodes at timings different from each other in each of M rows; the integrating circuit may be provided for each of M rows of the photosensitive section; and the differential amplifier circuit may sequentially input integral signals outputted from the respective integrating circuits provided for M rows of the photosensitive section and sequentially output respective signals corresponding to the intensities of optical signals inputted to the photodiodes arranged in the array of M rows and N columns. Here, the case where M=N=1 is the case were individual constituents exist one by one. The case where Mxe2x89xa72 and N=1 is the case where M photodiodes are arranged in one-dimensional array in the photosensitive section. The case where Mxe2x89xa72 and Nxe2x89xa72 is the case where M rows and N columns of photodiodes are arranged in a two-dimensional array in the photosensitive section. The differential amplifier circuit sequentially outputs respective signals corresponding to the intensities of optical signals inputted to the M rows and N columns of photodiodes in the photosensitive section.
The photodetector apparatus may further comprise a CDS (correlated double sampling) circuit between the integrating circuit and the differential amplifier circuit. The CDS circuit includes a second full differential amplifier, having first and second input terminals and first and second output terminals, for feeding the integral signals from the first and second output terminals of the integrating circuit into the first and second input terminals; a first capacitor disposed between the first input terminal and first output terminal of the second full differential amplifier; a first switch disposed in parallel with the first capacitor; a second capacitor disposed between the second input terminal and second output terminal of the second full differential amplifier; and a second switch disposed in parallel with the second capacitor; the first and second output terminals of the second full differential amplifier outputting respective fluctuations of the integral signals outputted from the first and second output terminals of the first full differential amplifier in the integrating circuit. Also, the differential amplifier circuit inputs the respective fluctuations of integral signals outputted from the first and second output terminals of the second full differential amplifier in the CDS circuit and outputs, according to a difference therebetween, the signal corresponding to the intensity of optical signal. The signal outputted from the differential amplifier circuit is only the one corresponding to the magnitude of the current signal outputted from the photosensitive section in this case as well since noise components cancel each other out, thus yielding a higher S/N ratio.
In the photodetector apparatus, the CDS circuit may comprise second reference potential setting means for setting each of the first and second output terminals of the second full differential amplifier to a reference potential before an operation for determining the fluctuations of integral signals. In this case, even though the respective potentials of two output terminals of the second full differential amplifiers may be unstable, each of the fluctuations of integral signals outputted from the second full differential amplifier becomes stable.
In the photodetector apparatus, the photosensitive section may have photodiodes arranged in an array of M rows and N columns and sequentially output respective current signals from N photodiodes at timings different from each other in each of M rows; the integrating circuit and CDS circuit may be provided for each of M rows of the photosensitive section; and the differential amplifier circuit may sequentially input respective fluctuations of integral signals outputted from the CDS circuits provided for M rows of the photosensitive section, and output respective signals corresponding to intensities of the optical signals inputted to the photodiodes arranged in the array of M rows and N columns. Here, the case where M=N=1 is the case were individual constituents exist one by one. The case where Mxe2x89xa72 and N=1 is the case where M photodiodes are arranged in one-dimensional array in the photosensitive section. The case where Mxe2x89xa72 and Nxe2x89xa72 is the case where M rows and N columns of photodiodes are arranged in a two-dimensional array in the photosensitive section. The differential amplifier circuit sequentially outputs respective signals corresponding to the intensities of optical signals fed to the M rows and N columns of photodiodes in the photosensitive section.